The inhalation of toxic substances in the environment remains a significant human health hazard. To understand and quantify toxic effects of inhaled air contaminants, it is necessary to determine the relationships between inhaled dose and observed health-related responses or effects in exposed humans or in experimental animals. The inhaled dose is calculated from the product of the airborne mass concentration, minute volume, exposure duration, and deposition efficiency. Deposition efficiency is estimated from mathematical models and laboratory experiments with similar airborne contaminants. There is one class of airborne contaminants, volatile aerosols, for which there is little or no information on deposition efficiency in humans or animals. This lack of information makes it difficult to make informed estimates of inhaled doses in human or animal exposures. Volatile aerosols are defined in this proposal as airborne systems having a significant fraction of the airborne mass in both the vapor and particle phases. Some examples of volatile aerosols include industrial chemicals (hydroquinone, glycerol, TNT), pesticides and herbicides (atrazine, ethion, carbofuran, and disulfoton), and "tear gas". The proposed work will bridge the gap between previous disparate disciplines relating to the deposition of vapors and the deposition of nonvolatile aerosols. The overall objective of this project is to relate the deposition efficiency of volatile aerosols in humans to physical properties of the airborne system, including exposure concentration, saturation vapor concentration, particle size, and blood/gas partition coefficient. The results of computer models will be compared to measured deposition efficiencies of volatile aerosols in humans. Unlike nonvolatile aerosols, the deposition efficiency of volatile aerosols is expected to change with exposure concentration. In many cases, the deposition efficiency of a volatile aerosol will be significantly different from that of a nonvolatile aerosol of the same particle size. Completion of the proposed theoretical and experimental studies will significantly increase our understanding of the respiratory tract deposition of volatile aerosols, an important but often overlooked class of airborne contaminants, and provide the information necessary for informed estimates of inhaled dose in humans.